Why are solid objects with refraction node so dark?
It's difficult to be sure without seeing your nodes for the material, but this is most likely due to the way the inner surface is reacting with the roughness of the refraction to refract beyond 90 degrees due to a combination of the angle of incidence of the incoming ray and the roughness. For example :



Depending on your requirement (whether you want the material to actually behave as 'refractive) you could either address this by using the Backfacing geometry input to mix between Refraction and Transparent (effectively hiding the backfaces) :



Or also use the Incoming for for the Normal so that rays are always refracted just for the roughness, not the actual surface angle :